This document discusses different ways to visualize categorical data, including contingency tables, bar plots, segmented bar plots, mosaic plots, and pie charts. It provides examples of how to use contingency tables to show the relationship between two categorical variables, and explains the differences between bar plots and histograms. Guidelines are given for choosing the appropriate proportion to examine when looking for a relationship between categorical variables. Finally, it notes some limitations of pie charts for comparing data across groups.

1. +
Working with Categorical Data
Slides edited by Valerio Di Fonzo for www.globalpolis.org
Based on the work of Mine Çetinkaya-Rundel of OpenIntro
The slides may be copied, edited, and/or shared via the CC BY-SA license
Some images may be included under fair use guidelines (educational purposes)

2. Contingency Tables
A table that summarizes data for two categorical variables is called a
contingency table.
The contingency table below shows the distribution of students' genders
and whether or not they are looking for a spouse while in college.

3. Bar Plots
A bar plot is a common way to display a single categorical variable. A
bar plot where proportions instead of frequencies are shown is called a
relative frequency bar plot.
How are bar plots different than histograms?
Bar plots are used for displaying distributions of categorical variables, while
histograms are used for numerical variables. The x-axis in a histogram is a
number line, hence the order of the bars cannot be changed, while in a bar
plot the categories can be listed in any order (though some orderings make
more sense than others, especially for ordinal variables.)

4. Choosing the
Appropriate Proportion
Does there appear to be a relationship between gender and whether
the student is looking for a spouse in college?
To answer this question we examine the row proportions:
● % Females looking for a spouse: 51 / 137 ~ 0.37
● % Males looking for a spouse: 18 / 70 ~ 0.26

5. Segmented Bar and Mosaic Plots
What are the differences between the three visualizations
shown below?

6. Pie Charts
Can you tell which order encompasses the lowest percentage of
mammal species? NO!
http://www.bucknell.edu/msw3

7. Comparing Numerical Data Across Groups:
side-by-side box plot